This invention relates to enameling and powder compositions used as an insulating coating material for transformer wire. One of the problems involved with current methods and materials used in manufacturing and applying transformer wire coatings is the need for a suitable solvent for dissolving the coating constituents and providing a low viscosity solution. Since the solvents currently utilize expensive hydrocarbon and cresols, other materials are continuously being evaluated in an attempt to reduce the quantity of solvents employed. Over the past 15 years for example, an insulating coating composition consisting of a mixture of polyvinyl acetal and phenolic was reduced in solvent from 85 weight percent down to 75 percent. This reduction was realized by variations in the polyvinyl acetal and phenolic materials as well as a selected combination of hydrocarbon and cresol solvents. Besides the expense involved in utilizing liquid solvents in the wire coating industry, requirements are now being made by the Environmental Protection Agency to reduce solvent usage by a substantial amount in order to reduce the overall concentration of solvents existing in the atmosphere.
U.S. Patent Applications Ser. No. 889,889 filed Mar. 24, 1978, now U.S. Pat. No. 4,215,174 and Ser. No. 595,034 filed July 11, 1975, now abandoned, disclose a three component wire insulating composition which includes an epoxy resin in combination with polyvinyl acetal and phenolic resins. The three component composition further reduced the solvent content down to 70 weight percent and less by taking advantage of the good film-forming properties of the epoxy resin. The use of an epoxy wire coating per se has not heretofore proven feasible due to the poor hydrolytic stability existing with known epoxy compounds. When transformer wires are coated for electrical insulating purposes, and are subjected to long exposure times in the presence of heat and moisture, it is essential that the coating remain electrically stable. Hydrolytic stability therefore is an important parameter for evaluating efficient transformer wire insulating materials. In order to determine hydrolytic stability, the transformer wire coatings are subjected to moisture and temperature for a prescribed period of time and are subsequently measured to determine whether the electrical insulating properties have deteriorated. Wires coated with epoxy compounds per se become hydrolytically unstable and are infeasible for long term transformer wire coatings.
Another requirement for transformer wire coating materials is a low dissipation factor. Since the electrical properties of the coating depend to a large extent upon the transformer operating temperature, the wire coating materials must be able to withstand the high temperatures involved under short circuit load conditions. In order for the transformer wire coating to be electrically and thermally stable, the dissipation factor, which is a fairly good indication of the ability of the coating to dissipate heat, must be determined at various operating temperatures. If the transformer wire coating has too high a dissipation factor, thermal runaway can occur causing insulation decrease to an inoperable value.
Formulations intended for use as insulating coatings must be carefully evaluated for temperature, moisture and overall electrical stability for long periods of time in order to ensure that short circuits do not occur due to electrical insulation failure. As described earlier, various epoxy resin compositions were hydrolytically unstable and unsuitable per se as wire coatings. Attempts to combine epoxy resins, phenolic resins, and polyvinyl acetal such as suggested within U.S. Pat. No. Re. 25,625 have not proven successful when evaluated for transformer wire coatings. Coatings prepared from the aforementioned re-issued patent were too inflexible to withstand the transformer winding operation. Wire coatings prepared from the adhesive composition disclosed within U.S. Pat. No. 3,239,598 resulted in wire coatings having an excessive dissipation factor and poor flexibility.